2.1 General Considerations
Polyfunctional macromolecules, such as proteins, can be purified by a variety of techniques. One of these techniques is known as ion-exchange chromatography. In ion exchange chromatography, proteins are separated on the basis of their net charge. For instance, if a protein has a net positive charge at pH 7 it will bind to a negatively charged ion-exchange resin packed in a chromatography column. The protein can be released, for example, by decreasing the pH or adding cations that compete for binding to the column with the positively charged groups on the protein. Thus, proteins that have a low density of net positive charge, and thus a lower affinity for the negatively charged groups of the column, will tend to emerge first, followed by those having a higher charge density.
Generally, the ion-exchange resins which are used in these procedures are solids possessing ionizable chemical groups. Two types exist: cation-exchangers, which contain acidic functional groups such as sulfate, sulfonate, phosphate or carboxylate, and a second type, anion-exchangers, which contain functional groups such as tertiary and quaternary amines. These ionizable functional groups may be inherently present in the resin or they may be the result of the chemical modification of the organic or mineral solid support.
Organic ionic-exchangers which are made from polysaccharide derivatives, e.q., derivatives of agarose, dextran and cellulose, etc., have been used for both laboratory and industrial scale ion-exchange chromatography. However, these ion-exchangers have many disadvantages. First, polysaccharide-derived ion-exchangers are not very mechanically stable and are not resistant to strong acids. This instability limits the length of the column and, also, limits the flow rate through the column.
Second, such ion-exchangers have limited sorption capacity due to the limited number of ionic or ionizable groups that can be attached to the polysaccharide.
Third, these polysaccharidic derivatives are poor adsorbents for use in rapid fluidized-bed separations because of the low density of the material. In a fluidized bed it is desirable to pass the fluid without simultaneously washing out the particles. Therefore, it is generally desirable to have as great a density difference as possible between the solid support particles (e.q., silica) and the fluidizing medium.
The intrinsic high density of inorganic sorbents based on passivated mineral substrates facilitates packing and rapid decantation into chromatographic columns. Dense packing prevents formation of empty spaces and channeling when using packed beds. On the other hand, fluidization of dense particles in aqueous suspension is possible at high flow rates that, in turn, are very desirable when dealing with large scale applications. Operation of fluidized beds at high superficial flow velocities is generally not possible with low-density organic or polymeric sorbents, which can be eluded from fluidized beds at relatively low liquid flow rates.
On the other hand, synthetic polymers are mechanically more stable than inorganic supports, and the former are more resistant to strong acidic conditions. However, they suffer disadvantages as well, such as limited capacity, limited solute diffusivity and thus, limited productivity. These synthetic polymers also suffer to some extent from the problem of non-specific adsorption of biomolecules, such as proteins. Untreated mineral supports such as silica are also inadequate in many chromatographic protein separation applications because of such non-specific adsorption.
Non-specific adsorption is caused by the interaction of a protein with the surface of the support--be it organic or inorganic in nature. For example, silica is an acidic compound, and the negatively charged silanol groups present at the solid/liquid interface tend to create a separate ion-exchange interaction between the surface of silica and the protein. Non-specific adsorption is also caused by hydrogen bonding that takes place between, e.g., amino groups present in the amino acid residues of proteins and these same silanols present at the silica surface. Such non-specific interactions create separation problems during chromatography--e.g., poor protein recovery and/or inadequate resolution. An important objective in the design of a chromatographic separation is generally to ensure a "single-mode" process of adsorption. However, the ion-exchange behavior associated with surface silanols can create a "mixed mode" adsorption system which makes the separation of biomolecules much more difficult. Although the sorption capacity generated by ionic silanol groups is low, the intensity of the interaction between the silanol groups and proteins can be high. These interactions therefore have the potential to cause denaturation of certain proteins.
Finally, both polysaccharides and most hydroxyl containing synthetic sorbents are sensitive to the cleaning solutions used in industrial settings, which often include strong oxidizing agents such as hypochlorite or peracetic acid and which may be characterized by extremes of pH.
Thus, there is an important need for the development of improved passivation methods for the treatment of the surfaces of both polymeric and inorganic chromatographic supports in contact with protein-containing solutions, which method is capable of preventing or minimizing such non-specific interactions between proteins and the chromatographic support in order to improve the efficiency of chromatographic processes.